mime type for JSON /api/metadata is currently text/plain

[karlcow]

A request to Interop 2022

Creates a request to the API for the metadata https://wpt.fyi/api/metadata?includeTestLevel=true&products=chrome%2Cfirefox%2Csafari

This JSON resource is sent with text/plain, not sure it is expected or just something which fell through the cracks. I don't think it breaks anything.

:status: 200
Content-Type: text/plain; charset=utf-8
Date: Wed, 24 May 2023 00:38:02 GMT
Content-Encoding: gzip
Content-Length: 340114
Cache-Control: private
Vary: Accept-Encoding
x-cloud-trace-context: 7726ecda59e4c97b091d7dc33c3625a7
Server: Google Frontend
Strict-Transport-Security: max-age=31536000; preload